1. Field of the Invention
This invention is directed to electrochromic devices having an effective amount of an electrochromically-inert reducing or oxidizing additive in the electrolyte to eliminate a separate step of changing the initial state of reduction or the state of oxidation of an electrochemically active layer, such as by ionic intercalation of the electrochemically active layer of those devices. This invention is further directed to electrochromic devices wherein the electrochemically active layer of the device is comprised of a mixture of an electrochemically active material and an electrochromically-inert reducing or oxidizing additive to eliminate the separate step of changing the initial state of reduction or the state of oxidation of an electrochemically active layer, such as by ionic intercalation of the electrochemically active material. The invention is also directed to processes of preparing the above-described electrochromic devices as well as a process for preparing an electrochemically active layer which does not require a separate step of changing the initial state of reduction or the state of oxidation of an electrochemically active layer, such as one involving ionic intercalation.
2. Related Prior Art
It is well known that light transmission and reflection can be variably controlled by electrochromic (EC) devices. It would be advantageous to use large area EC devices as variable transmission windows to reduce the energy consumption in buildings and automobiles. Moreover, EC devices are used for displays, dynamic optical filters and automotive mirror applications. Thus, a continuing need exists for EC devices which can be produced in a cost effective manner.
A conventional EC device is comprised of two opposing substrates having electronic conductors coated on the inward facing surface of the substrates. At least one of the conductor coated substrates is transparent, and for an EC window device both conductor coated substrates are transparent. Typically, the inward facing surface of each conductor is coated with an electrochemically active material. The substrates are assembled to form a cell which is filled with an electrolyte that is in conductive contact with both layers of electrochemically active material. Devices of this type are disclosed, for example, in U.S. Pat. No. 4,750,816, U.S. Pat. No. 4,938,571, U.S. Pat. No. 5,080,471, U.S. Pat. No. 5,189,549, and U.S. Pat. No. 5,215,821.
However, prior to assembling such electrochromic devices it has been often necessary to either reduce or oxidize the electrochemically active material such as by intercalation of a layer of electrochemically active material with ions by either chemical or electrochemical means. This step is employed to either reduce the electrochemically active material if cations are used or oxidize the material if anions are used. This is done to ensure that the electrochemically active materials in an electrochromic device having more than one type of electrochemically active material are all in a substantially equivalent state of light transmission, i.e., they are either colored or clear.
For example, where the two layers of electrochemically active material are tungsten oxide and vanadium oxide it is known to intercalate the latter with Li.sup.+ ions prior to assembling the device. Similarly, U.S. Pat. No. 5,215,821 discloses a device having electrochemically active material layers of tungsten oxide and Prussian blue, wherein the tungsten oxide is electrochemically reduced to a blue tungsten bronze by exposing the tungsten oxide electrode under negative potential to an acidic solution prior to assembling the device. This electrochemical intercalation of the tungsten oxide allows for the extraction of the protons from the tungsten oxide to intercalate the Prussian blue electrode after the device is assembled and a voltage is applied so as to obtain a Prussian blue of high transparency.
An EC device having polyaniline and tungsten oxide electrochemically active material layers is also known. Under ambient conditions polyaniline exists in its oxidized state and is colored. However, under ambient conditions, tungsten oxide is in its oxidized state, which is transparent. Tungsten oxide can be colored by injection of electrons (reduction) with concomitant intercalation of cations, such as Li.sup.+ or H.sup.+. As noted previously, in order for such an electrochromic device to function properly, both layers of electrochemically active material must be in a substantially similar state of light transmission, i.e., when polyaniline is colored the tungsten oxide is colored or when polyaniline is clear the tungsten oxide is clear. This can be accomplished by reducing the polyaniline or the tungsten oxide, as appropriate prior to device assembly.
It is known to reduce the polyaniline coating by the separate step of intercalating the polyaniline with protons or by extracting anions therefrom prior to assembling the device. Upon reduction, the polyaniline becomes transparent. However, when stored under ambient conditions the polyaniline gradually oxidizes and acquires color. The tendency of polyaniline to oxidize creates processing disadvantages for such electrochromic devices since the reduced polyaniline must either be quickly assembled into the device or stored and/or processed under inert conditions.
The intercalation of ions into an electrochemically active material may also be required in other electrochromic device constructions. In particular, EC devices having only one electrochemically active electrode, can be constructed where the electrochemical activity in the electrochemically active layer is balanced by a redox active material capable of electrochemical activity in the electrolyte, such as disclosed in U.S. Pat. No. 4,671,619. A device of such construction having a polyaniline electrochemically active layer is disclosed in the U.S. Patent Application entitled "Electrochromic Devices" application Ser. No. 08/547,578, filed Oct. 24, 1995, now U.S. Pat. No. 5,729,379, which was a continuation-in-part of U.S. patent application Ser. No. 08/329,502, filed Oct. 26, 1994, the disclosure of each of which is incorporated by reference herein as if fully set forth. Such a device, however, may also suffer from the processing disadvantage that the electrochemically active polyaniline will revert to its oxidized state prior to assembling the device if not stored under inert conditions.
An object of this invention is to provide an electrochromic device having an electrochemically active layer that does not require a separate step of changing the initial state of reduction or the state of oxidation of the electrochemically active material, such as by ion intercalation.
Another object of this invention is to provide a process for preparing an electrochemically active layer on a substrate which does not require a separate processing step of initially oxidizing or reducing the electrochemically active material, such as by ion intercalation.
A further object of this invention is to provide a process for preparing an electrochromic device having an electrochemically active material which has been reduced or oxidized without the use of a separate processing step.